1. Field of the Invention
The present invention relates to a process for producing a polyacrylonitrile membrane for use in reverse osmosis, and more specifically to a process which comprises preparing a membrane-forming solution of an acrylonitrile polymer dissolved in formamide (FA)-containing dimethylformamide (DMF); forming the polymer solution into a membranous product while maintaining the temperature of the polymer solution within a specified range; removing the solvent under a specified solvent removal condition (solidifying condition); subjecting the thus-obtained solidified membrane to a heat treatment and further to a plasma treatment, whereby a polyacrylonitrile reverse osmotic membrane is obtained which is excellent both in membranous separation ability and in mechanical strength, and is asymmetric and porous in structure.
2. Description of the Prior Art
In recent years, in the wide field of applications including waste water treatment, desalination of sea water, food industry, medical treatment, etc., separation methods for various substances using semi-permeable membranes are given attention and are expected for their further development.
As such membranous separation methods, there have been proposed various techniques such as ultrafiltration, reverse osmosis, etc. depending on the particle size of the substance which is suspended, dispersed or dissolved in the aqueous medium to be treated. It goes without saying that the properties required for the membrane should be different depending on the respective objects of separation, but the properties commonly required for them are a high permeation rate of the aqueous medium, an excellent ability for rejection molecules having a particular molecular weight, a sufficient mechanical strength and chemical resistance.
Although separation membranes produced from acrylonitrile polymers are said to have excellent properties as materials for separation membranes, the membranes that have been put into practical use at present are only those for use in the separation of macromolecules, namely those in the field of ultrafiltration, microfiltration or dialysis, and they are scarcely applied to the field of reverse osmosis for separating low molecular substances such as sodium chloride. This is considered to result from the fact that, although acrylonitrile polymer membranes have a sufficient water permeability and salt rejection ability, it is difficult for them to form a membrane structure, especially a skin layer, having a sufficient strength to withstand the severe separating operation in reverse osmosis. That is to say, although it is indispensable to form such a skin layer in order to obtain a reverse osmotic membrane having a sufficient water permeability and salt rejection ability, it has not been successful up to this time to find a technical means to produce a membrane having such a skin layer in an industrially advantageous manner.